This looks somewhat promising. It's a SAW VCO with thru-zero FM. So far just a proof of principle. But quite simple and cost effective. None of that doo-dah with rectifying the CV, separately reversing the core direction, etc. Just modulate the core's input current back and forth and carefully process what comes out...

I'm currently working on improving the implementation before more careful characterization. Right now it has 8 opamps plus the OTA. Two of the opamps are for deriving the correct time-reversed waveform from what comes out of the core (which is an interesting signal on its own.)

are the improvements you mention (partly) related to the way the fm input tends to clip/distort (it seems like it does, am I wrong) ?

Yes, thanks for pointing that out. It looks like I was overdriving something there -- probably pushing past the end of the frequency range. There are some levels that need to be looked at more closely.

This project is moving along bit by bit. I was stuck for a while getting the bipolar discharge switch comparators working right, but finally it seems OK. Still lots of work to do, but for now here are a couple of demos.

The first has sawtooth carrier and triangle fm modulation, in unison. Three sections: (1) no modulation, (2) modulation just slightly into the negative frequency regime and (3) deep TZ modulation.

The second has a 3/2 frequency ratio. Modulation is either by triangle or square wave. Various depths of dynamic modulation in several patterns.

Thanks for all the interest! This is beginning to look like it might fly.

This just keeps getting better and better. I was afraid it would be hard to get good tracking with the control current passing through an OTA. It took almost a whole day, but I got it working right yesterday. It runs up to over 100 kHz and tracks to within 0.2% up to 20 kHz. Better than a lot of standard VCOs out there. Wheee ...

Okay, I'll buy. You and your oscs said all you freakin' need to say to get my vote. How soon do you think we can expect a PCB/project? (I'm not trying to rush you, I just want to know what to expect--I have no idea how to guess and I'm curious [

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